A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC via a projection system. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of photo-sensitive material (e.g., photo-resist) provided on the substrate. In general, a single substrate will contain adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic apparatus in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between a final element of the projection system and the substrate. In an example, the liquid is distilled water or ultra-pure water, although another liquid/fluid can be used. Suitable fluid may be a wetting fluid, an incompressible fluid and/or a fluid with higher refractive index than air, desirably a higher refractive index than water. Fluids, excluding gases, are particularly desirable. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g., quartz) suspended therein, or a liquid with a nano-particle suspension (e.g., particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
In an immersion lithographic apparatus, immersion fluid is handled by a fluid handling system, fluid handling device, fluid handling structure or fluid handling apparatus. The above expressions may have been referred to throughout the literature in the same field as a fluid confinement structure, an immersion hood or a shower head. In an example, the fluid handling system may supply immersion fluid and therefore be a fluid supply system. In an example, the fluid handling system may at least partly confine immersion fluid and thereby be a fluid confinement system. In an example, the fluid handling system may provide a barrier to immersion fluid and thereby be a barrier member, such as the fluid confinement structure as previously mentioned. In an example, the fluid handling system may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid relative to the substrate. The flow of gas may form a seal to confine the immersion fluid so the fluid handling structure may be referred to as a seal member; such a seal member may be the fluid confinement structure as previously mentioned. In an example, immersion liquid is used as the immersion fluid. In that case the fluid handling system may be a liquid handling system. In reference to the aforementioned description, reference in this paragraph to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.
The throughput of the immersion lithographic apparatus is limited by its critical scan speed, which may be determined by the properties of the photo-resist or top coat on the substrate and the characteristics of the fluid. A scan speed is a relative speed of a moving object with respect to the fluid handling system. The object may be, for example (but not limited to), the substrate, a sensor mounted on a moveable table, such as a measurement table, or a top surface of the moveable table, etc. In the case of the substrate, the critical scan speed is defined as the maximum scan speed at which the substrate can be moved relative to the projection system and/or fluid handling system without losing the liquid, i.e. without compromising the confinement of the liquid. The critical scan speed may also be determined by the geometry and operating parameters of the immersion lithographic apparatus, such as the shape, size and design of the fluid handling system and scan length and the flow rate of fluids of the immersion lithographic apparatus.
Taking the substrate as an example, when the immersion lithographic apparatus operates at a scan velocity above the critical scan speed, liquid tends to be lost at a side of the substrate that is receding from the fluid handling system, i.e., the receding side of the substrate with respect to the fluid handling system. The lost liquid will typically form droplets on the substrate. The evaporation of the liquid droplets will apply a local cooling load on the substrate, resulting in local deformation of the substrate, which in turn, leads to overlay error. For the immersion lithographic apparatus, the term “overlay error” quantifies the misplacement and misorientation of the lithographically projected pattern relative to the desired location and orientation. Liquid droplets are also undesirable because un-evaporated droplets may cause bubbles in the liquid as a result of trapped ambient gas, e.g., air. The bubbles may affect imaging quality. Loss of the liquid needs to be avoided and, therefore, limits the scan speed of the immersion lithographic apparatus, in particular for more hydrophilic photo-resists, which have lower contact angles to the liquid and therefore typically requires lower scan speeds than more hydrophobic photo-resists. Similarly, a hydrophobic surface (such as a surface of the sensor mounted on the moveable table which may be illuminated through the liquid) has a higher contact angle to the liquid and therefore has a higher critical scan speed than a more hydrophilic surface. Liquid loss may also occur at crossing edges, stickers and gaps, such as a gap between an outer edge of the substrate and a substrate table that is moveable with respect to the projection system or a crossing between the substrate table and the measurement table. At this type of crossing, the contact line of the receding meniscus may get pinned at sharp edges of e.g., stickers, and often these surfaces are more hydrophilic than the photo-resist coated substrates.